Power amplifiers may be employed in a wide variety of electronic systems. For example, power amplifiers may be employed in transmitters to generate transmit signals. Transmitters may be employed in a wide variety of electronic systems including mobile communication devices.
A power amplifier may be characterized by an output power and an efficiency. The efficiency of a power amplifier may be defined as a ratio of the output power of the power amplifier and an amount of DC power consumed at an output of the power amplifier.
It may be desirable in a variety of electronic systems to employ a power amplifier having a relatively high efficiency. For example, a high efficiency power amplifier for a transmitter in a mobile communication device may reduce power consumption and increase battery life.
It may also be desirable in a variety of electronic systems to vary the output power of a power amplifier. For example, it may be desirable in a mobile communication device to increase the strength of a transmit signal when the mobile communication device is far away from a base station and to decrease the strength of the transmit signal when the mobile communication device is close to a base station.
A power amplifier having a variable amount of output power may exhibit its maximum efficiency at a predetermined amount of output power. For example, a power amplifier in a transmitter for a mobile communication device may be designed for maximum efficiency at its maximum output power. Unfortunately, an amplifier designed for maximum efficiency at its maximum output power may exhibit a substantially reduced efficiency at lower amounts of output power. A lower efficiency at low power may waste the DC supply power in an electronic device and decrease battery life in a mobile device.